1. Field of the Invention
The present invention relates to an information terminal used in a content playing back system. And that the content playing system shares rights of content acquired from, for example, a content server by a plurality of information terminals to enable playback by the respective information terminals.
2. Description of the Related Art
In recent years, there has been prevalent a distribution service for downloading rich content such as audio content from a content server to an information terminal device such as a cellular phone or a personal computer. In the device utilizing this kind of service, the downloaded content are temporarily stored in a memory, and the stored content are read out and played back from the memory in response to a user's playing back operation.
In some cases, rights information for protecting a copyrights or the like is assigned to content. The kind of content are encoded and stored, and, at the time of playback, the encoded content are decoded and played back under a condition specified by rights information. The playback condition includes, for example, a playback count or a playback period. As an encoding system, there is used, for example, a system of encoding content by a content key made of random numbers or a system of encoding the content key by a binding key. Such a binding key is generated by, for example, specific identification information of a device of content and a secret key of the device. By using such an encoding system, content are obtained in a specific device bound state, whereby playback of content can be limited to a device having the content encoded therein.
In the meantime, recently, there have been proposed a variety of devices for sharing a rights of the acquired content with another person and transferring or dividing and assigning the rights to such another person. For example, techniques for transferring a rights include: transferring content bound by specific identification information of a device from a device serving as a transfer source to another device serving as a transfer destination and transferring the specific identification information via a secure transmission channel, thereby enabling use of the specific identification information by a device serving as a transfer destination after transferred and disabling use of the specific identification information by the terminal serving as a transfer source (refer to, for example, Jpn. Pat. Appln. KOKAI Publication No. 2003-303137).
Further, a content distribution standard using a PKI (Public Key Infrastructure) is standardized as a method for sharing rights. For example, a Digital Rights Management v2 specification or the like standardized by an Open Mobile Alliance (TM) can be exemplified. In this specification, content are defined by encoded content and a rights object that includes rights information for using the encoded content and a key used at the time of encoding content. The encoded content are distributed from a rights server, and the associated rights information is distributed from the rights server to a client server, respectively. In addition, the client device has a pair of public keys that is composed of certificates with a secret key and a public key, and the rights information is encoded by using a public key of the client device.
In this standard, a content sharing function called a domain is introduced. This function regards a set of the client devices registered in the rights server as a domain, and can share content as long as they are in the domain. A specific mechanism is configured as follows, for example.
That is, the client device registers a domain by sending the client device's certificate to the rights server. The rights server issues a domain key encoded by the client device's public key from the rights server after checking validity of the client server's certificate. The client device decodes the domain key by the secret key, and securely stores it. Further, rights information for domains is encoded by using the domain key described previously, and the client device acquires participating rights information for domains, thereby decoding a content key by using the securely stored domain key. Further, content of a plain text can be acquired by decoding content by using the content key. As a result, the acquired content can be played back.
However, the technique disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2003-303137 disables use of specific identification information by a terminal serving as a transfer source with transfer of the rights of content. In the device serving as the transfer source as well, there is a problem that a change of the specific identification information is unavoidable. On the other hand, it is necessary to register a method for sharing content using a domain in the rights server. For this reason, there is a problem that sharing content and selecting a rendering device cannot be easily carried out by a user's intention and preference.